Welcome to New to Linux Info
Distributions
Linux differs from other OS's because we have a thing called distributions shortend to distros.
This concept is alien to you Windows refugees. Linux is mostly open source, that means that developers can develop their own incarnation of Linux and release it to the general public, legally. This is made possible because of the modular design of Linux.
Click one of the links to the left to grab yourself a copy from the more popular developer's web site.
Live Distro's
These are a great concept. Again new to ex Windows users. Live distros are full blown working distros that you can run straight from you CD rom allowing you to test and try out a distro to your hearts content. When you are satisfied, finshed playing/testing you reboot and hey presto its gone. No damage at all to your installed OS. This is because it loads some of the OS into memory and reads the rest from the CD. Some of the live distros even allow you to install them to your hard drive right from the desktop when you are happy.
The Kernel
The Kernel is the heart of the OS. Linux's Kernel is constantly being updated, get yours from the link below.